1. Technical Field
The present invention relates to a method for communicating (transmitting & receiving) digital image signal, a digital image communication (transmission & reception) device, a digital image transmitter and a digital image receiver enabling long distance communication.
2. Background Art
In the past, for example, as a method for sending a digital image signal to a display device such as an LCD monitor, a projector, etc. from a digital image reproducing device such as a computer, video image reproducing equipment, etc., metal wire transmission by coaxial cable was widely used.
As an interface for sending the digital image signal, the DVI (Digital Visual Interface) standard is used, and in the recent years, the HDMI (High Definition Multimedia Interface) standard, an extension based on the DVI standard including voice signal communication, is also proposed.
In these standards, a TMDS (Transition Minimized Differential Signaling) method is used for communication of image signal.
In these digital transmission standards, problems such as fluctuation or blurring of the picture, inaccuracy of color development, etc., which were conspicuous in prior analogue transmissions, are reduced.
However, digital transmission standards by metal wire transmission in high frequencies from several hundred MHz to several GHz have a problem in that image quality of the middle resolution class (XGA; 1024×768 pixel) can be communicated only up to around 10 m and image quality of the high resolution class (HDTV; 1920×1080 pixels) can be communicated only up to around 5 m.
Accordingly, for example, as shown in FIG. 25, for maintaining the image quality, a repeating driver 105, namely a repeater, comprising an amplifier, etc. is interposed between a transmission driver 102 for transmitting the digital image signal from digital image output equipment 101 such as a computer and a reception amplifier 104 for receiving the digital image signal transmitted from the transmission driver 102 and outputting the digital image signal to digital image input equipment 103 such as a display device connected with the transmission driver 102 and the reception amplifier 104.
In the DVI standard, DDC (Display Data Channel) CLOCK, DDC DATA, HPD (Hot Plug Detect) are respectively used as DDC signals for transmitting control information to the digital image output equipment 101 from the digital image input equipment 103. In FIG. 25, a transmission driver 111 transmits the DDC signals to the digital image output equipment 101 from the digital image input equipment 103; a reception amplifier 112 receives the DDC signals from the transmission driver 111; and a repeating driver 113 is interposed between the transmission driver 111 and the reception amplifier 112 and repeats the DDC signals.
The transmission driver 102 for transmitting digital image signal and the reception amplifier 112 for receiving the DDC signals constitute a transmission section 116, the repeating drivers 105 and 113 constitute a repeater 117, and the reception amplifier 104 and the transmission driver 111 constitute a reception section 118.
The digital image output equipment 101 and the transmission section 116, and the digital image input equipment 103 and the reception section 118 are respectively connected by DVI cables 120, and the transmission section 116 and the repeater 117, and the reception section 118 and the repeater 117 are respectively connected by coaxial cables 106 for every signal.
Furthermore, as shown in FIG. 26 or 27, a method for using fiber optic cables as transmission paths and an optical transmission device conforming to the specifications of the DVI standard and transmitting digital image signals is also proposed (for example, Japanese Unexamined Patent Application Publication No. 2002-366340).
In the optical transmission devices shown in FIGS. 26 and 27, image signals (R, G, B) and a reference clock signal (CLK) output from the digital image output equipment 101 are converted into optical signals for every signal at the transmission section 121 via a laser driver 122 and a laser diode 123, and the optical signals are transmitted for every signal by respectively corresponding fiber optic cables 124.
The respective optical signals received by a reception section 125 are respectively re-converted into electric signals by a photodiode (PD) 126 and a PD amplifier 127, and input into a terminal for DVI of the digital image input equipment 103.
The DDC signals are, in the case shown in FIG. 26, transmitted by metal wire such as coaxial cable. In the case shown in FIG. 27, the DDC signals are transmitted as optical signals (image signals) from the digital image input equipment 103 to the digital image output equipment 101 by a laser driver 127a, a diode 127b, a photodiode 122a and a PD amplifier 122b suitably prepared for communicating the DDC signals.
However, as shown in FIG. 25, in a communication device using a repeater, a power source is required respectively for the transmission section 116, the repeater 117 and the reception section 118, and a coaxial cable such as a BNC is required for each image signal of R, G, B, and CLK. Therefore, a complete system using this communication system is complex and costly due to the number of required components. Also, because of the number of components, the communication device is inconvenient to instal and use.
In such high frequency signal communication using metal wiring, EMI (electro-magnetic interference) would be a problem, and in long distance transmission, it would be possible that the problem becomes conspicuous.
Meanwhile, as shown in FIGS. 26 and 27, in an optical communication device transmitting the image signal by optical communication, a plurality of fiber optic cables or a cable formed by binding a plurality of fiber optic cables in one bundle is used. Such optical communication is costly and difficult to instal and use because a plurality of fiber optic cables/bundels is required.
Accordingly, in the present invention, such problems are considered and an object is to provide a digital image communication device, a digital image transmitter and a digital image receiver enabling long distance communication of a digital image signal at low cost and with high quality.